Environmentalists have targeted the oil-producing tar sands in Canada in part because its crude comes with heftier carbon emissions than conventional sources. Now, a new study in the Proceedings of the National Academy of Sciences (PNAS) has found an additional source of carbon that has been unaccounted for: peatlands. Mining the oil in the tar sands, dubbed "oil sands" by the industry, will require the wholesale destruction of nearly 30,000 hectares of peatlands, emitting between 11.4 and 47.3 million metric tons of additional carbon.

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Once destroyed peatlands will not return note the researchers: "Constraints imposed by the postmining landscape and the sensitivity of peatland vegetation prevent the restoration of peatlands that dominated the premining landscape." Instead drained peatlands will be turned into upland forests, which will store considerably less carbon.

"Claims by industry that they will 'return the land we use—including reclaiming tailings ponds—to a sustainable landsca that is equal to or better than how we found it' and that it 'will be replanted with the same trees and plants and formed into habitat for the same species' are clearly greenwashing," the researchers write.

Already carbon emissions from the tar sands produce significantly more carbon than conventional sources with various research showing around 20 percent higher than conventional oil to three times higher. However, such estimates have not included the loss of carbon due to peatland destruction, which the researchers estimate will be equal in total to "seven years worth of carbon emissions by mining and upgrading (at 2010 levels)."

A recent study has found that if the entirety of the tar sands were exploited it would raise global temperatures 0.64 degrees Fahrenheit (0.36 degrees Celsius). This represents around 45 percent of how much the world has warmed since the Industrial Revolution.